1. Field of the Invention
The present invention generally relates to optical communication systems, and more particularly, the present invention relates to a preamplifier and an optical receiving device that may be utilized in an optical communication system.
2. Description of the Related Art
FIG. 12 is a block diagram of a conventional optical receiving device 101 coupled to an optical fiber 102. As shown, the optical receiving device 101 includes a light receiving element 103, a preamplifier 104, a threshold setting limiter amplification circuit 105, and a clock/data identification recovery circuit 106.
The light receiving element (RCR) 103 converts an optical signal transmitted from the optical fiber 102 into an current signal. The light receiving element 103 may be implemented, for example, by a positive intrinsic negative-photo diode (PIN-PD) or an avalanche photo diode (APD).
The preamplifier (P-AMP) 104 converts the current signal output from the light receiving element 103 into a voltage signal, and amplifies the voltage signal. The preamplifier 104 may be implemented, for example, by a trans-impedance amplifier (TIA).
The threshold setting limiter amplification circuit (THRES AMP) 105 compares the voltage signal output from the preamplifier 104 with a preset threshold voltage to thereby generate an output signal corresponding to logic “1” (high level) or logic “0” (low level). The logic “1” and “0” voltage amplitudes of the output signal are independent of the power-level of the received optical signal. The threshold setting limiter amplification circuit 105 may be implemented, for example, by a limiting amplifier (LA).
The clock/data identification recovery circuit (RECOV CKT) 106 functions to recover a clock signal (CLOCK) and a data signal (DATA) synchronized with the clock signal from the signal output from the threshold setting limiter amplification circuit 105. The clock/data identification recovery circuit 106 may be implemented, for example, by a clock and data recovery (CDR) circuit.
Although not shown, one or more optical amplifiers (for example, an Erbium doped fiber amplifier (EDFA)) may be interposed in the input optical transmission path, i.e., between an optical transmitting device (also not shown) and the optical receiving device 101.
FIG. 13A is a waveform diagram (eye pattern) of a non-amplified optical signal inputted to the optical receiving device 101 (i.e., in the simplified case where an optical amplifier has not been implemented in the optical transmission path). FIG. 13B is a graph illustrating a statistical noise probability distribution of logic “1” (high level) and logic “0” (low level) of the optical signal of FIG. 13A.
In FIG. 13B, where the vertical axis denotes optical power, standard deviations σ1 and σ0 of the noise probability distribution of logic “1” and “0” are approximately equal to each other. As such, an optimal threshold level for distinguishing between logic “1” and logic “0” lies halfway between center values of logic “1” and “0,” that is, at a level of 50% power (where the logic “0” level is at 0%, and the logic “1” level is at 100%).
When the optical signal of FIG. 13A is input to the optical receiving device 101, the optical signal is first converted into a current signal in the light receiving element 103. The current signal is converted into a voltage signal in the preamplifier 104 from which a linearly amplified electrical signal is output if the current signal has a small amplitude. On the other hand, if the current signal has a large amplitude, a saturated electrical signal is outputted from the preamplifier 104. In either case, the optimal threshold at an output of the preamplifier 104 is located a middle level between logic “1” and “0,” that is, a center value of the output amplitude.
The threshold setting limiter amplification circuit 105, which receives the output signal from the preamplifier 104, sets a threshold with an absolute value level of an amplitude of an output voltage of the preamplifier 104. Typically, the preamplifier 104 is capacitively-coupled to the threshold setting limiter amplification circuit 105. If the optimal threshold is a center value of the input amplitude, a threshold set in the threshold setting limiter amplification circuit 105 becomes about 0 mV and is independent of the output amplitude of the preamplifier 104.
FIG. 14A is a waveform diagram (eye pattern) of an amplified optical signal inputted to the optical receiving device 101 (i.e., in the case where an optical amplifier has been implemented in the optical transmission path). FIG. 14B is a graph illustrating a statistical noise probability distribution of logic “1” (high level) and logic “0” (low level) of the optical signal of FIG. 14A.
As shown in FIG. 14B, a standard deviation σ1 of the noise probability distribution of logic “1” is larger than a standard deviation σ0 of the noise probability distribution of logic “0”. This is due to spontaneous emission of noise proportional to optical power by the optical amplifier. Since a noise distribution (variance) of logic “1” is larger than the noise distribution of logic “0,” an optimal threshold level for distinguishing between logic “1” and logic “0” is biased towards logic “0.”
When the optical signal of FIG. 14A is input to the optical receiving device 101, the input optical signal is ultimately converted into an amplified voltage signal, as described above. However, if the preamplifier 104 executes an ideal linear operation (equivalent amplification), an optimal threshold for an output waveform of the preamplifier 104 is constant when expressed by a percentage (with a logic “0” level as 0% and a logic “1” level as 100%) independent of received optical power. However, the optimal threshold is proportional to average power of an input optical signal when it is expressed by an absolute value of an amplitude of an output voltage.
The threshold set in the threshold setting limiter amplification circuit 105 has an absolute value level as described above. For example, if the threshold is 40% (when expressed by a percentage with a logic “0” level as 0% and a logic “1” level as 100%), the threshold set in the threshold setting limiter amplification circuit 105 becomes −10 mV if single phase output amplitude of the preamplifier 104 is 100 mV, and becomes −20 mV if this amplitude is 200 mV.
If the threshold is 40% (by percentage) and a gain (trans impedance gain) of the preamplifier 104 is 1 KΩ, the relationship between the threshold set in the threshold setting limiter amplification circuit 105 and the power of the optical signal is shown in FIG. 15. Also, the relationship between the threshold-set in the threshold setting limiter amplification circuit 105 and average current of the light receiving element 103 is shown in FIG. 16. It can be seen from FIG. 16 that the threshold set in the threshold setting limiter amplification circuit 105 is proportional to the average current of the light receiving element 103.
Since the threshold to be set in the threshold setting limiter amplification circuit 105 is proportional to the average current of the light receiving element 103, the optical receiving device 101 generally employs a type of feedback control. In the feedback control, a threshold is determined in accordance with the current flowing through the light receiving element 103 and is set in the threshold setting limiter amplification circuit 105.
In such an optical communication system, in the case where a PIN-PD is used as the light receiving element to realize a transmission rate of 10 Gbps, power of an input optical signal required for the optical receiving device 101 is −20 dBm to +1 dBm, and its dynamic range is 42 dB (125 times) in terms of voltage. In addition, when the optical amplifier is used, the power of the input optical signal required for the optical receiving device 101 exhibits a characteristic in which a penalty due to noises included in the optical signal (an external factor to narrow a dynamic range of the power of the input optical signal, which is not originated from the optical receiving device) is deducted from the requirement.
In addition, since input sensitivity of a limiting amplifier used as the threshold setting limiter amplification circuit 105 is commonly 10 mVpp in single phase and the preamplifier 104 requires output amplitude of 20 mVpp in single phase in order to cause the clock/data identification recovery circuit 106 to identify clocks or data without any error, the preamplifier 104 requires a gain of 1 KΩ or so.
However, when an optical signal of +1 dBm is input to the preamplifier 104 that performs a linear operation (equivalent amplification) and has the gain of about 1 KΩ, since the preamplifier 104 has output amplitude of about 2.5 Vpp in single phase, there arises a problem in that a typical limiting amplifier has amplitude exceeding a rating.
To overcome this problem, a preamplifier has been developed which is equipped with an automatic gain control (AGC) function to automatically lower a gain of the preamplifier when a large optical signal is inputted thereto.
In addition, there has been proposed a technique in which a filter is provided to adjust an electrical signal, which is amplified by the preamplifier 104 in order to increase reception sensitivity in long distance transmission, to a variable pass band. For example, reference is made to Japanese Patent Application Publication No. 2006-81141 (JP '141), esp. paragraphs [0023] to [0030] and FIGS. 1 and 2 thereof.
In addition, there has been proposed a technique in which an output of a preamplifier is clamped to a constant voltage in order to respond to a sudden change of received optical power. For example, reference is made to Japanese Patent Application Publication No. Hei6-152535 (JP '535), especially paragraphs [0013] to [0016] and FIG. 1 thereof.
FIG. 17 shows a relationship between a threshold set in the threshold setting limiter amplification circuit 105 and an average current of the light receiving element 103 when the preamplifier 104 equipped with the above-mentioned conventional automatic gain control function. In FIG. 17, it is assumed that a threshold of an input optical waveform is 40t by percentage and a gain of the preamplifier 104 when a small signal is inputted thereto is 1 KΩ.
Although the optical receiving device 101 using the preamplifier 104 equipped with an automatic gain control function can attain a favorable error characteristic, since a range of the set threshold of the threshold setting limiter amplification circuit 105, which is proportional to a current of the light receiving element 103, and a current range of the light receiving element 103 at which the automatic gain control function begins to operate are varied with change of IC power or operation temperature, as shown in FIG. 17, or it is difficult to control the set threshold in an range of automatic gain control operation, there arises a problem in that a programmable control IC or the like has to be used for a peripheral circuit, thereby necessitating difficult threshold control.
In addition, the technique of JP '141 suffers a drawback in that it requires a filter having a variable capacitance diode and a voltage controllable circuit to control a voltage applied to the variable capacitance diode. Thus, the control and circuitry are relatively complex.
In addition, the technique of JP '535 has a disadvantage in that it can not attain good identification recovery of a signal since a low level and a high level of an output waveform are clipped so as to output a waveform having vertically-symmetrical constant amplitude.